GREELEY — The 3.4-magnitude earthquake that rattled Greeley and the northern Weld County region May 31 was the first notable seismic activity in the northeast corner of Colorado in nearly 45 years.

And with the abundance of oil and gas exploration and production in Weld County in the past decade-plus — along with injection wells that coincide — it’s not hard for some to make a tentative link between the relatively isolated seismic event and the burgeoning energy industry.

According to experts, that link to the energy industry is legitimate. Well, sort of.

Movers and shakers

Matt Morgan, senior research geologist with the Colorado Geological Survey, a state government agency within the Colorado School of Mines, said the May 31 earthquake — about four miles northeast of Greeley — was the third-largest northeastern Colorado quake on record. On May 26, 1969, a 4.2-magnitude event was recorded about 15 miles east of Greeley.

As for the largest quake in the region, it was also the largest in Colorado history, according to Morgan. He said the estimated 6.6-magnitude earthquake on Nov. 8, 1882, “rang church bells in Denver and was felt as far east as Salina, Kan.

“It was probably located about 35 miles west of Greeley near Estes Park, although one of the original researchers suggested this event could have been located somewhere north of Denver and east of Boulder,” he added.

Morgan said scientists and geological groups are still improving on “seismic networks” in Colorado, but it’s safe to say it is not one of more active states in terms of seismic events. In fact, it’s not even in the top 10, according to figures from the United States Geological Survey.

Although some data is more than a decade behind, figures of earthquakes nationwide from 1974 to 2003 show Colorado as 14th on the list overall, with its 24 earthquakes during that span making up only one-tenth of a percent of the nation’s overall seismic activity.

And while a lot has changed over the past 10 years, some experts say it could be what happened before the oil and gas boom — not since — that is behind the latest earthly shifts.

A seismic whodunit

The energy industry has been linked to seismic activity within the state before.

According to a January 2011 report from the Colorado Oil and Gas Conservation Commission (COGCC), the most-notable case of earthquakes triggered by injection of fluids into the earth’s subsurface — what scientists call “induced seismicity” — came in the 1960s when a series of earthquakes were ultimately linked to a 12,000-foot injection well drilled at the Rocky Mountain Arsenal near Denver. The injection well was drilled in March 1962 and was followed by a series of earthquakes from January 1963 to August 1967.

When injection stopped in 1968, so, too, did the quakes.

But in that same report, the commission contends that, in accordance with federal laws and the commission’s rules and policies, there are now “safeguards in place to reduce the likelihood of induced seismicity,” including regulations dictating injection volume and pressure, and regular input from the Colorado Division of Water Resources and the Colorado Geological Survey.

The report goes on to say that injection wells must utilize a well construction method of cementing surface and production casing, and injection well zone water must be sampled prior to drilling to ensure against contamination.

That doesn’t change the fact that injection wells have, in the past couple of years, been strongly linked to seismic activity in states such as Oklahoma and Ohio.

“The forces to drive (an) earthquake are natural, but human action triggers the event either by adding to the forces or by weakening the rock,” said CGS Senior Geologist Paul Morgan — no relation to colleague Matt Morgan. “Injections wells fall into this category.

“We carefully monitor scientific data from studies that are relevant to this and other problems,” Morgan added. “There have been studies for a few years indicating that there is a spatial association among hydraulic fracturing, fluid injection and earthquakes. Most of the earthquakes have been small, however, and industry has often reduced its activity on a voluntary basis.”

Morgan also pointed out that some quakes in Colorado, particularly those on the Western Slope, could be related to a different arm of the energy industry: Mining.

“Some (earthquake) causes are 100-percent human-induced,” Paul Morgan said. “Many of the events in the west-central part of the state are in areas of coal mining.”

Those events, referred to by Morgan as “rock bursts,” are generally shallow quakes, and the May 31 Greeley quake — at 7.8 kilometers deep — does not fall into that category.

“At this point it is unknown if the Greeley event was natural or human-induced through wastewater injection,” Matt Morgan said. “Unlike the injection-induced (4.0-magnitude) Youngstown, Ohio, event that was quite shallow (about 1.4 miles or 2.3 kilometers deep), the Greeley event was located about 5 miles down.

“This would suggest a natural event, but more data is needed to determine its trigger,” he added.

Digging deeper

There’s a chance that scientists might never find the cause of the recent quakes in Greeley (a second, smaller quake was recorded a day after the May 31 rattler). And both Paul and Matt Morgan said the CGS does not have a budget that allows for lengthy investigations.

Furthermore, Matt Morgan said the four seismometers spread over the state and tied to the USGS seismic network do not do well in measuring smaller seismic events, which are often tied to bigger ones and give researchers a better idea of seismic patterns that could show a trend in seismic activity. There’s also the additional challenge of having records that only go back to 1973 and measure only events of a 3-magnitude or greater, added Paul Morgan.

“Many areas in Colorado, including Greeley, do not have earthquake recorders nearby and the threshold for detecting earthquakes may be relatively high,” Paul Morgan said. “The word of caution, therefore, is that the Greeley earthquake is an isolated event since at least 1973 and of a magnitude of 3 or greater. Smaller earthquakes may not have been detected or felt.”

“Until a better seismic network is permanently located in the state, we will know little about their causes or sources,” added Matt Morgan.

But Matt Morgan said help is on the way.

“The University of Colorado-Boulder is working on deploying a local seismic network that can measure smaller events and potentially locate hitherto unknown subsurface faults,” he added.

Morgan said Colorado has about “90 mapped faults that cut geologic deposits that are less than 2 million years old.

“Undoubtedly, there are many more that we do not know about that are masked by very young surficial cover or located in the subsurface,” he said. “Be assured, we are on the lookout for those features and will continue to provide information about any new discoveries to the citizens of Colorado.”

And with the largely global focus on natural gas exploration and production, it’s a sure bet everybody else will be watching, too.